1. Field of the Invention
The invention is directed to a method for determining the orientation of an external magnetic field with giant magneto-resistor (GMR) sensors. The invention is also directed to a system with GMR sensors for determining the orientation of an external magnetic field.
2. Description of the Related Art
GMR sensors were first manufactured in the 1980's. They are distinguished by their high sensitivity of their electrical specific resistance to the orientation of an external magnetic field.
GMR sensors have proven themselves as ideal sensors for non-contacting position and speed measurement of magnetic objects and for the non-contacting measurement of electrical currents. Due to their ruggedness when confronted by harsh environmental influences, they are often utilized in an industrial setting or in the field of automotive electronics. Since the resistance of the GMR sensors is dependent on the orientation of the external magnetic field and, within a broad range, not on the strength of the magnetic field, the GMR sensor can be easily employed for measuring field orientations without the distance between the magnetic source and the GMR sensor having to be exactly adjusted.
There are various embodiments of GMR sensors. FIG. 1 illustrates an embodiment of a GMR sensor illustrated that is composed of very thin anti-ferromagnetic layers that are alternately stacked with very thin, conductive, non-magnetic layers 1-2, for example copper. The uppermost and the bottom layer, i.e., the covering layers 1-1 of the GMR sensor, are composed of a soft anti-ferromagnetic material whose magnetic orientation already aligns in accord with soft, external magnetic fields 1-5 that penetrate the material. The other layers lying between the conductive, non-magnetic layers 1-2 are hard anti-ferromagnetic layers 1-3 that, for example, are composed of cobalt. A magnetic direction can be impressed in hard anti-ferromagnetic layers using a very strong external field, this direction not changing even given moderately strong external fields with a different orientation. The orientation of the anti-ferromagnetic, hard layers 1-3 is impressed once with a very strong external magnetic field (>15 kA/m) and determines the orientation of the internal magnetic field of the GMR sensor and, thus, the internal orientation of the GMR sensor. In order to nonetheless change the internal orientation of the GMR sensors, an external magnetic field of at least the same strength must be applied again.
The variation of the electrical resistance due to a variation of the orientation of an external magnetic (soft) field is based on the fact that the resistance that electrons encounter in magnetic materials is dependent on the angle between spin directions of the electrons and the magnetic field direction of the material. Electrons that have assumed the spin direction of the external magnetic field 1-5 in the soft covering layer 1-1 encounter a low electrical resistance in the hard magnetic layers 1-3 when the internal magnetic field 1-6 of the GMR sensor has the same magnetic orientation as the external magnetic field 1-5. The electrical combined resistance, R, of a GMR sensor is thus composed of a fundamental resistance R0 and an angle-dependent resistance R1=½ΔR(1−cos α), so that the following equation applies:R(T·α)=R0(T)+½ΔR(T)×(1−cos α).  (1)                 where α is the angle between internal orientation 1-6 of the GMR sensor and the orientation of the external field 1-5 in the plane of the covering layers 1-1, and ΔR(T) is the maximum magneto-resistive resistance of the GMR sensor.        
The parameter T incorporated in Equation (1) indicates that the resistance is somewhat dependent on the temperature of the GMR sensor. Typically, the fundamental resistance increases given increasing temperature, whereas the magneto-resistive resistance drops with the temperature. Typical values for the relative temperature drift of the fundamental resistance R0(T) are approximately 0.05%/K through 0.2%/K and approximately −0.05%/K through −0.2%/K for the relative temperature drift of the magneto-resistive resistance. This temperature dependency causes an angle allocation from a measured resistance to be ambiguous when the temperature varies and is unknown.
Typically, the layers of the GMR sensors are only a few nm thick and are applied onto small substrates with precision techniques, for example sputtering methods. The structures of the GMR sensors are applied onto a substrate with lithographic methods so that a plurality of GMR sensors can also be applied onto the substrate in a small space. The electrical connections between the GMR sensors are often designed in a meandering fashion in order to generate defined resistances on the leads to the GMR sensors.
FIG. 2 shows a simple measuring structure with a GMR sensor of the Prior Art with which the orientation of an external magnetic field can be identified in the plane of the GMR sensor layers without temperature compensation. The measuring structure is composed of a current source 2-1 with a set current I, of a GMR sensor 2-2 with an internal orientation 2-5, as well as of a voltmeter device 2-3 that takes the voltage drop-off V=V(P2)−V(P1) across the GMR sensor at the points P1 and P2. When R0 and ΔR of the GMR sensor are known from preceding calibration measurements, then the angle α between external magnetic field 2-4 and internal orientation 2-5 can be calculated from the measured voltage drop-off with the assistance of Eq. (1). This simple measuring structure, however, has the following disadvantages:                1) the angle measurement is unambiguous only in the region of 180 degrees but not in the full 360 degree range;        2) the magneto-resistive effect, ΔR/R0, generally amounts to only a few percent, so that the voltage measurement is superimposed by a large offset voltage Voffs=R0×I; and        3) the temperature dependency of the parameters R0(T) and ΔR(T) does not allow an unambiguous angle determination given an unknown temperature of the GMR sensor.        
A significant improvement for avoiding these difficulties is represented by the employment of two series-connected, structurally identical GMR sensors with internal orientation anti-parallel to one another (FIG. 3a and FIG. 3b). FIG. 3a shows the two GMR sensors 3-2 and 3-3 with anti-parallel internal orientation and the current source 3-1 with the set current I1 that flows off to the current sink 3-6 via the points P3, P2 and P1. The external magnetic field 3-4 acting on the GMR sensors describes the angle α with the internal magnetic orientation of the GMR sensor 3-2. The external magnetic field should be the same for both GMR sensors, which is easily met when the spatial extent of the two GMR sensors together is small compared to the spatial structure of the external magnetic field 3-4.
When the anti-parallelism of the GMR sensor 3-3 to the reference GMR sensor 3-2 is taken into consideration, then the following derives from Eq. (1) and FIG. 3a: V(P2)−V(P1)=(R0(T)+½ΔR(T)×(1+cos α))×I1  (2) and V(P3)−V(P2)=(R0(T)+½ΔR(T)×(1+cos α))×I1  (3) 
The difference V1 of the two voltages then supplies:V1=V(P2)−V(P1)−V(P3)+V(P2)=ΔR(T)×I1×cos α  (4) 
The employment of two series-connected anti-parallel GMR sensors can thus be utilized by difference formation to eliminate the great (and temperature-dependent) voltage offset R0(T) that occurs in FIG. 2. The difference formation can be realized in a simple way with analog electronic circuits to prevent time delays that occur due to digitalization and digital calculations.
In order to eliminate the ambiguity of the angle determination in the 360 degree range, a second circuit of the type of FIG. 3a may be driven in parallel with the same current source. Such a second circuit is shown in FIG. 3b. Such electronic circuits with GMR sensors for the measurement of the orientation of a magnetic field can, for example, be derived from the German patent document DE 196 19 806. The second circuit comprises GMR sensors 3-10 and 3-11 with the same structure as the first circuit, with the difference that the internal orientations of the two GMR sensors are perpendicular to the farthest-reaching extent to the internal orientations of the GMR sensors of the first circuit. The two circuit must lie close enough together that they essentially encounter the same external magnetic field 3-4. The current source 3-12 generates the current I2 that flows off to the current sink 3-13 via the points P3′, P2′ and P1′. The voltages V(P2′)−V(P1′) and V(P3′)−V(P2′) that form at the points P1′, P2′ and P3′ are subtracted from one another as in FIG. 3a, so that—analogous to Eq. (4)—a second angle-dependent voltage measurement is obtained withV2=V(P2′)−V(P′)−V(P3′)+V(P2′)=ΔR(T)×I2×sin α.  (5) 
By division of Eq. (5) by Eq. (4), a temperature-independent relationship is obtained between the angle α and the measured voltages V1, V2, V1′ and V2′:α=arctan(ΔV′/ΔV)=arctan((V1′−V2′)/(V1−V2)),  (6)                 where the currents I1 and I2 are assumed to be the same for the sake of simplicity. In order to assure the unambiguousness in the entire 360 degree angle determination, a circuit must be additionally utilized that decides on the basis of V1, V2, V1′ and V2′ whether the angle is situated in the range between −π/2<α<+π/2 or in the range −π<α<−π/2 or, respectively, +π/2<α<+π.        
FIG. 4 shows such an electronic circuit of the Prior Art upon employment of the GMR sensor circuits described in FIG. 3a and FIG. 3b, these being connected in parallel to one another, i.e. P1=P1′ and P3=P3′. The first GMR sensor 4-4 and the series-connected, second GMR sensor 4-5, both of which have anti-parallel internal magnetic orientation relative to one another, form the first sensor sub-circuit 4-2; the first GMR sensor 4-6 and the series-connected, second GMR sensor 4-7, which likewise have anti-parallel internal magnetic orientation, form the second sensor sub-circuit 4-3. The internal magnetic orientation of the first sensor sub-circuit 4-2, which is established by the internal magnetic orientation of the first GMR sensor 4-4, is also vertical to the internal magnetic orientation of the second sensor sub-circuit, which is established by the internal magnetic orientation of the first GMR sensor 4-4. The orientation of the external magnetic field 4-14 describes the angle α with the orientation of the internal magnetic field of the first GMR sensor 4-4 of the first sensor sub-circuit 4-2.
Both sensor sub-circuits are supplied by the same current source 4-1. Since the four GMR sensors have essentially the same resistance parameters, the current divides equally onto the two sensor sub-circuits. The two operational amplifiers 4-10 and 4-11 form the difference between the signals V(P2)−V(P4) and V2=V(P2′)−V(P4), so that the output signal is freed of offset in the way described above:V1=V(P2)−V(P4)=½ΔR(T)cos α×I/2  (7) and V2=V(P2′)−V(P4)=½ΔR(T)sin α×I/2  (8). 
The ratio V2/V1 supplies an unambiguous, temperature-independent relationship between angle α and the two measured output signals:α=arctan(V2/V1)  (9). 
One disadvantage of the relationship indicated in Eq. (6) and Equ. (8), however, is that the calculation of the arctan cannot be accomplished with a simple analog circuit. An approximate linearization is also only possible in a small value range. The calculation therefore generally requires a digitalization of the two output signals with a subsequent calculation of the arctan. The digitalization of the two values, however, is both cost-intensive as well as time-consuming. The digital calculation of the arctan also requires programming, since this function is not implemented in standard micro-controller circuits. This also leads to time delays. Finally, the tangent exhibit infinities in the value range that are handled poorly with a computer.